U.S. Pat. No. 6,539,707 (JP 2002-235584A) proposes an air-fuel ratio feedback control system. In this system, two catalysts, an upstream catalyst and a downstream catalyst, are provided in series in an exhaust passage of an internal combustion engine to purify exhaust emissions. A first exhaust sensor is provided upstream the upstream catalyst to detect a first air-fuel ratio there. A second exhaust sensor is provided intermediate the two catalysts to detect a second air-fuel ratio there. A third exhaust sensor is provided downstream the downstream catalyst to detect a third air-fuel ratio there. A target second air-fuel ratio to be attained in or downstream the upstream catalyst, which is indicated by an output of the second exhaust sensor, is determined based on the detected third air-fuel ratio actually attained in or downstream the downstream catalyst, which is indicated by an output of the third exhaust sensor. A target first air-fuel ratio, which is to be attained upstream the upstream catalyst, is determined based on both of the target second air-fuel ratio and the detected second air-fuel ratio. An air-fuel ratio of mixture is feedback controlled by controlling a fuel injection amount based on a difference between the target first air-fuel ratio and the detected first air-fuel ratio.
With recent tighter exhaust emission regulations, an oxygen storage amount (oxygen occluding amount) in the downstream catalyst is increased. It is therefore likely in the conventional air-fuel ratio control system that the amount of lean gas components such as NOx emitted into the atmosphere increases under the condition that the upstream catalyst is in the rich condition (little oxygen) and the downstream catalyst is in the lean condition (much oxygen). This occurs for the following reasons.
When the upstream catalyst is in the rich condition, rich components such as unburned HC and CO flow from the upstream catalyst. If the amount of occluded oxygen in the downstream catalyst is small, the lean condition will readily end because the rich components flow from the upstream catalyst to the downstream catalyst. If the amount of occluded oxygen in the downstream catalyst is large, on the other hand, the lean condition will not readily end even if the rich components flow from the upstream catalyst. As a result, as shown in FIG. 8, lean components such as NOx emitted into the atmosphere will increase when the oxygen occlusion in the downstream catalyst is large (solid line) than when it is small (dotted line.
In the conventional air-fuel control system, further, the target first air-fuel ratio is feedback controlled in the lean direction even when the downstream catalyst is still in the lean condition at the time of a change in the upstream catalyst from the lean condition to the rich condition. Therefore, a sufficient amount of rich components for ending the lean condition in the downstream catalyst cannot be supplied to the downstream catalyst. As a result, the downstream catalyst tends to remain in the lean condition for a longer period and discharge the lean components such as NOx into the atmosphere.